Here's a daft, but pragmatic suggestion for both including the common Black Cab (aka London Taxi) and making use of the tram system:
Fudge a tram system with a "black cab" tram.
The tram lines are almost invisible anyway, and having black cabs for trams could look pseudo-realistic. Lowish capacity, so you'd need lots of them. To avoid too many driver's wages, have two cabs running nose to tail as a single "tram", one driver, carries 12....
I hope the purists aren't offended too much, but it seemed a possible way to bludgeon the CiM tram system into a non-tram city. I'd be a little worried by the tram's "offroading" abilities, but hey, I'm just trying to get folk thinking.
You could add a black cab to the bus list, but then what are you going to do for the trams? Trolley buses for historic periods maybe, but surely not for modern times? Or will the entire tram system get disabled for the period?
And for realism, you'd want lots of cyclists darting about in the traffic. Huge traffic jams are also a must. Nothing must travel at more than 10mph. (15km/h). Speed cameras should flash if they do. (Fines could become an expense as well as an income!)
And have the police close all roads over half the map when there is an accident of any sort...... (jaded lol)